CDMA (code-division multiple access) systems are well known. See, generally, CDMA Cellular Mobile Communications and Network Security, Dr. Man Young Rhee, Prentice Hall 1998, ISBN 0-13-598418-1, and standard TIA/EIA/IS-95, hereinafter "IS-95".
In CDMA transmission between a base station and a plurality of mobile stations, all transmissions share all the bandwidth all the time. (Typically, all the transmissions from mobile stations to base share one band, and all transmissions from base to mobiles share another band.) Each mobile station's data stream is multiplied by binary codes (pseudonoise codes, or PN codes) in order to spread its spectrum, and typically is orthogonally modulated (as by Walsh-code modulation) to transform it from a series of bits into a series of PN "chips". Broadband signal received at the base station is converted from radio frequency back to baseband. At the base station signals from a plurality of mobile stations are, in effect, summed.
To decode the data stream from a particular mobile station, it is necessary to "correlate" the spread-spectrum signal with the same codes that the particular mobile station used for spreading its data stream. For this purpose, the base station is equipped with the same kinds of code generators as the mobile station and is able to produce "local replicas" of each base station's PN codes.
It remains to get the local replicas synchronized with the codes in a received signal. The received signal is subject to transmission delays, and the transmission delays may vary, especially if the mobile user is in motion. Furthermore, the received signal may include several multipath components from a mobile user. State-of-the-art base stations employ "rake" receivers which correlate each of the multipath components and recombine them, requiring that the transmission delay of each one be determined.
A base station typically includes an apparatus known as a "searcher" which can determine the delay of a signal from a mobile user, or under multipath conditions the delays of several multipath components of a signal from a mobile user, by attempting to correlate through a range of various amounts of delay and determining the amount of delay at which the best correlation occurs. The range of delay over which attempts are made is known as the "uncertainty region" and is chosen to include the range of greatest expected delay.
A typical state-of-the-art searcher can determine the transmission delay of a component within one-half of a PN chip duration. The searcher depicted in FIG. 1, for example, employs a plurality of correlators to correlate the signal against various delay amounts of the local replica PN codes against the input signal, each sequence being delayed from the previous one by an amount "delta". Most of the correlations will be "noise" except for those that correspond to the signal (including a multipath component) coded with that PN code which will produce a value above the noise. The transmission delay of each component can then be determined with a granularity of "delta" taken as one-half a PN chip duration for the present discussion.
A signal correlated against a perfectly synchronized local replica would yield the maximum signal-to-noise ratio (SNR) attainable from that signal, and SNR would decrease as synchronization decreases. For example, for a discrepancy of one-half a PN chip in determining delay, SNR could decrease up to 2.5 db. To maximize performance, particularly regarding SNR of received signals, there is a need to determine transmission delay with accuracy significantly greater than one-half of a PN chip duration.
It is also necessary to "track" the transmission delay of the components, as it may vary during a user's call, particularly if he is in motion. With reference to pages 901 et. seq. of Spread Spectrum Communications Handbook, M. K. Simon, J. K. Omura, R. A. Scholz and B. K. Levitt, Second Edition, 1994, one finds that there are two PN tracking loop configurations in predominant use, namely, the delay-locked loop (DLL) and tau-dither loop (TDL). Both of them fall within the class of early-late gate type loops in that the received PN code is correlated either simultaneously or alternately with delayed and advanced version of the receiver local code PN generator output to produce the error correcting characteristic. Both are complex and calculation-intensive.
The tracking problem is compounded by frequency-selective fading, which degrades the performance of the prior-art tracking loops. In addition, new requirements of emerging systems with intensive data traffic are adding considerable complexity, and therefore more system cost. There is thus a need for a delay tracking loop that introduces little complexity and cost.
It is thus an object of the present invention to determine the transmission delay of a spread-spectrum signal to an accuracy of at least within one-eighth of a PN chip duration at low cost and with low complexity.
It is a further object of the present invention to track the transmission delay of an ongoing spread-spectrum signal at low cost and with low complexity.
These and other objects of the invention will become apparent to those skilled in the art from the following description thereof.